Wednesday, 24 September 2014

Some time ago I published an article on "The Cartographic Journal" regarding a method to automatically change the light azimuth in shaded relief representations.
This method was based on clustering the aspect derivative of the DTM. The method was developed originally in R and then translated into ArcGIS, with the use of model builder, so that it could be imported as a Toolbox.
The ArcGIS toolbox is available here: www.fabioveronesi.net/Cluster_Shading.html

Below is the Abstract of the article for more info:

Abstract
Manual shading, traditionally produced manually by specifically trained cartographers, is still considered superior to automatic methods, particularly for mountainous landscapes. However, manual shading is time-consuming and its results depend on the cartographer and as such difficult to replicate consistently. For this reason there is a need to create an automatic method to standardize its results. A crucial aspect of manual shading is the continuous change of light direction (azimuth) and angle (zenith) in order to better highlight discrete landforms. Automatic hillshading algorithms, widely available in many geographic information systems (GIS) applications, do not provide this feature. This may cause the resulting shaded relief to appear flat in some areas, particularly in areas where the light source is parallel to the mountain ridge. In this work we present a GIS tool to enhance the visual quality of hillshading. We developed a technique based on clustering aspect to provide a seamless change of lighting throughout the scene. We also provide tools to change the light zenith according to either elevation or slope. This way the cartographer has more room for customizing the shaded relief representation. Moreover, the method is completely automatic and this guarantees consistent and reproducible results. This method has been embedded into an ArcGIS toolbox.

Basically it replicates all the equations and methods presented in the paper (if you cannot access the paper I can send you a copy privately). The script loads a DTM, calculates slope and aspect with the two dedicated functions in the raster package; then it cluster aspect, creating 4 sets.
At this point I used RSAGA to perform the majority filter and the mean filter. Then I applied a sine wave equation to automatically calculate the azimuth value for each pixel in the raster, based on the clusters.
Zenith can be computed in 3 ways: constant, elevation and slope.
Constant is the same as the classic method, when the zenith value does not change in space. For elevation and slope, zenith changes according to weights calculated from these two parameters.
This allows the creation of maps with a white tone in the valleys (similar to the combined shading presented by Imhof) and a black tone that increases with elevation or slope.